Not Applicable
1. Field of the Invention
This invention is in the field of methods and apparatus used to remove a xe2x80x9cwindowxe2x80x9d or section of piping from a casing pipe in an oil or gas well.
2. Background Art
Section milling of pipe, that is, removing a section of pipe installed down hole in an oil or gas well, by milling it away, has been known in the art for a long time. However, passing a section milling tool through a smaller diameter pipe in order to section mill a larger diameter pipe farther downhole has always been more difficult, and the known methods have not met with much success. Typically, the procedure has relied upon an attempt to mill the larger diameter pipe from above, proceeding in the downhole direction. In milling downwardly, the weight of the drill string, possibly including drill collars, is used to apply downward force to the mill to cause it to progress through the pipe being milled. This application of force to the mill by weight applied from above creates a wobble in the milling work string, which has a tendency to fracture the cutting inserts on the section mill blades. This, in turn, causes the mill to wear out sooner, resulting in the removal of less pipe footage before replacement of the mill is required. Further, since milling progresses downwardly, cuttings must be removed from the well bore as they are formed, to avoid forming a ball of cuttings around the mill and reducing its effectiveness. Specialized formulation of milling fluid, and maintenance of proper fluid flow rates, are required in order to circulate the cuttings out of the hole.
One example of a situation in which these section milling problems are important is in the resolution of a gas migration problem. Many oil and gas well producers are faced with the problem of wells that have gas migration between casing strings, and this gas may ultimately migrate back uphole to the wellhead system. This leakage could pose a serious problem in that the gas could be ignited, causing a well explosion. Consequently, in the interest of safety, such wells must be repaired. In doing so, it is generally considered necessary to provide a means of removing one or more inner strings of casing pipe, at a location downhole, and exposing an outer string of casing pipe for cementing, to seal off the gas migration path.
As an example, a 16xe2x80x3 cased hole may have a 10xc2xexe2x80x3 casing and a 7xe2x80x3 casing inside, in a more or less coaxial arrangement. Gas migration may occur between the 10xc2xexe2x80x3 casing and the 16xe2x80x3 casing. Heretofore, the typical repair has been to pilot mill all the 7xe2x80x3 and 10xc2xexe2x80x3 casings completely away, from the top, down to a selected location downhole. A packer is then set against the 16xe2x80x3 casing, and cement is installed on top of the packer. This is a time consuming and costly endeavor. Further, management of cuttings, cuttings disposal, and milling mud properties all have to be planned for in this program.
The method and apparatus of the present invention provide a better solution to this problem, as described in the following. In a first embodiment, a section mill is used in combination with an up-thruster tool and a downhole motor. The apparatus is tripped into the hole to position the section mill at the lower end of the downhole interval where a window is to be cut. The section mill is at or near the bottom of the apparatus, with a stabilizer, an up-thruster, a mud motor, and an anti-torque anchor positioned above that, in order. A spiral auger with a left hand twist can be positioned below the section mill, to assist in moving the cuttings downhole.
The anti-torque anchor is set against the innermost casing, the mud motor is run, and an upward force is exerted on the section mill with the up-thruster. The casing is cut through, and a portion of the casing is milled out, as the mill progresses upwardly. When the up-thruster reaches its full travel, the apparatus is released and re-set at a higher location, with the mill positioned at the upper end of the milled opening, and with the up-thruster extended. The process is then repeated. After milling of the desired window, other operations through the window can take place, such as cementing.
In a second embodiment, the same type of section mill is used in combination with an up-thruster tool and a rotating work string. The difference between this and the first embodiment is that the mill is rotated by a rotating work string, rather than a downhole motor, and no anti-torque anchor is needed. Here again, a spiral auger with a left hand twist can be positioned below the section mill.
Use of this invention increases the life of the mill, resulting in the milling of more footage with each mill, reducing the number of trips of the work string, and reducing rig costs. In either embodiment, the work string is always in tension while milling. Cuttings can be left down hole, which eliminates the need for special mud and the need for handling and disposing of the cuttings. A relatively constant force is exerted on the cutting blades. Pump pressure is regulated to keep a regulated upward force on the cutter, by means of the up-thruster. Better centralization of the drilling string and the cutter are achieved, with less wobble. Especially in the mud motor embodiment, there is much less wobble in the work string than with downward milling. Where used, the anti-torque tool eliminates back torque and results in a stiffer milling assembly. Drill collars are not needed; smaller pipe and smaller rigs can be used. Coil tubing can even be used in the downhole motor embodiment.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: